Factors governing the transition from active remediation of petroleum contamination in the subsurface to natural source zone depletion

Petroleum releases remain a major cause of soil and groundwater contamination worldwide, entraining significant resources to remediate. Active remediation methods often face diminishing returns over time, usually reaching an endpoint in a few years, as mass is depleted and petroleum composition changes leaving less mobile and recoverable relative saturations. Prolonged active remediation efforts challenge sustainability metrics based on energy use and greenhouse gas production. Intrinsic biodegradation, volatilization and dissolution collectively deplete petroleum light non aqueous phase liquid (LNAPL) mass, commonly referred to as Natural Source Zone Depletion (NSZD). Natural source zone depletion is typically evaluated for weathered petroleum releases, often after active recovery options have been exhausted. This study focuses on key factors that govern the transition to management of petroleum impacted sites via NSZD - i.e., when NSZD rates (commonly after weathering) adequately surpass what can be achieved by recovery methods. Based on decades of field monitoring, measurement, and modelling, the paper identifies key factors controlling the relative magnitude of active recovery versus NSZD mass removal rates leading to management via NSZD, and offers a conceptual roadmap towards addressing gaps in knowledge and understanding to allow confident transition to NSZD.